BALTIMORE, MARYLAND--Space physicists here at the spring meeting of the American Geophysical Union conceded today that Earth's high atmosphere contains numerous splotches of water. But a sharp disagreement persists over where this mysterious moisture is coming from.

Louis Frank, a prominent space physicist at the University of Iowa, has been arguing for more than 10 years that house-size, icy comets pummel Earth 20 times a minute, leaving their watery mark in the outer atmosphere. Planetary science, he insists, must be fundamentally revised to account for these small comets. Frank's colleagues, who have been skeptical about the existence of these splotches of water, now agree that he is seeing something new, but they are not willing to accept his revolutionary interpretation.

Frank, a protégé of radiation-belt discoverer James Van Allen, came up with the small-comet hypothesis in 1986 after finding tiny black spots in images returned by the orbiting Dynamics Explorer spacecraft. The spots were silhouetted against the ultraviolet glow of Earth's upper atmosphere, suggesting they were clouds of ultraviolet-absorbing water vapor. From their darkness and abundance, he inferred the existence of 10-meter balls of fluffy, snowlike ice that fall apart 1000 to 2000 kilometers above Earth as they streak in at 65,000 kilometers per hour.

At the meeting, Frank presented new images from cameras on the Polar satellite launched last year that seem to confirm his Dynamics Explorer observations. In the old images, the spots had been no larger than a single picture element, thousands of which are arrayed to form an image. Other researchers concluded that these single-pixel spots were nothing but instrumental noise, so much snow on an ultraviolet television. But Polar's finer pixel size revealed spots 10 to 20 pixels across. Frank's Polar camera also picked up the same spot in consecutive exposures as the spot moved across the field of view, and a second Polar ultraviolet confirmed another sighting, he says.

Atmospheric physicist Donald Hunten of the University of Arizona agrees that Frank has confirmed the existence of the dark spots. But the idea of small comets "certainly isn't valid," he says. "It is very easy to put forward five objections to the small-comet explanation, any one of which rules it out." Two of the biggest problems, say Hunten and many of his colleagues, are the huge amounts of water small comets should have left in the upper atmosphere and around the inner solar system--none of which has been found--and the apparent impossibility of smashing small comets into the moon and Earth without anybody noticing. At night, a small comet's atmospheric entry "would light up the whole sky," he says.

So what, if not small comets, is making the dark spots? No one, except Frank, is willing even to speculate. But it shouldn't be long before scientists begin offering alternatives. There's nothing like hard, believable data to fire the imagination.